Logic board without power

i7 3.4 GHz with no power

We recently performed an in‑depth diagnosis on an iMac logic board (820‑3299‑A / i7 3.4 GHz). Initial suspicion pointed to a power rail issue. The board was removed for bench testing.

Visual inspection

First, we look for discoloration, shifted components, damaged pads and traces. This often reveals heat damage or previous rework.

Resistance measurements

We measured resistances across multiple rails to locate shorts and open circuits, and to identify affected chipsets and unaffected areas. Heatsink, CPU and RAM were temporarily removed to simplify measurements.

After measuring many voltages, we swapped the CPU for further tests. This ruled out CPU faults and confirmed a defective power‑management chipset on the logic board.

Component replacement

The defective chipset was removed. We wicked away old solder and installed a new chip so that USB ports and slots would receive correct voltages again. After reflow and cleaning, we verified that GPU, RAM, CPU and other subsystems got the intended power levels.

CPU, thermal paste, heatsink and RAM were then reinstalled. The board passed initial bench tests before reassembly.

Testing

We performed repeated boots, functional checks and graphics tests. A small chipset fault can take down the whole machine — in this case no port or component received sufficient power. Fortunately there were no follow‑up damages, and the iMac was returned to the customer the same week.

Frequently asked questions

What is logic board 820‑3299‑A?

The main board of a 27‑inch iMac model with an i7 3.4 GHz CPU. It connects all components and distributes power.

How do you recognize a bad chipset?

Typical signs include missing power on USB and other I/O. Systematic resistance and voltage checks help localize the faulted chip.

How long does a chipset repair take?

Usually 3–4 hours including diagnostics, replacement and testing, depending on damage extent.